>env - PERL_SIGNALS=unsafe strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:39:02 2015 pid=31774: helper starts
Sun May 24 20:39:02 2015 pid=31773: main starts
Sun May 24 20:39:02 2015 pid=31773: main waiting for helper to lock
Sun May 24 20:39:03 2015 pid=31773: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:39:12 2015 pid=31774: helper ends
Sun May 24 20:39:12 2015 pid=31773: main flock done
Sun May 24 20:39:12 2015 pid=31773: main ends

>grep -C5 flock trace.txt
fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:39:03 2015 pid=317"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigaction(SIGALRM, NULL, {0x7fdcf683dde0, [], SA_RESTORER|SA_RESTART, 0x7fdcf5a2f670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43
rt_sigreturn()                          = 73
flock(3, LOCK_EX)                       = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:39:12 2015 pid=317"..., 52) = 52
alarm(0)                                = 0
close(3)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0

>env -  strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:43:14 2015 pid=31859: helper starts
Sun May 24 20:43:14 2015 pid=31858: main starts
Sun May 24 20:43:14 2015 pid=31858: main waiting for helper to lock
Sun May 24 20:43:15 2015 pid=31858: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:43:20 2015 pid=31858: main can't lock: Interrupted system call (error code 4)
Sun May 24 20:43:20 2015 pid=31858: main flock done
Sun May 24 20:43:20 2015 pid=31858: main ends
Sun May 24 20:43:24 2015 pid=31859: helper ends

>grep -C5 flock trace.txt
fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:43:15 2015 pid=318"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigreturn()                          = -1 EINTR (Interrupted system call)
rt_sigprocmask(SIG_BLOCK, [ALRM], [], 8) = 0
rt_sigaction(SIGALRM, NULL, {0x7f43b0eaede0, [], SA_RESTORER, 0x7f43b00a0670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43

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